Dual component polyurethane systems are widely used as coatings and in the preparation of elastic floor coverings. Moreover, other processes are known in which the inner surfaces of molds are sprayed with two component polyurethane systems. European Patent Application No. 0 189 845 teaches the preparation of cellular or noncellular plastics, such as coatings, from at least two starting materials which react with one another, such as organic polyisocyanates, higher molecular weight polyhydroxyl compounds and optionally lower molecular weight chain extending agents; whereby the starting materials are fed separately into a mixing zone, then mixed together, subsequently discharged and fed together with air heated to from 100.degree. C. to 120.degree. C. with the component mixture exiting the mixing zone. By way of illustration under these reaction conditions, a reaction mixture for preparing polyurethane coatings can be uniformly prepared and applied in an evenly arranged stream onto a substrate. A rapid increase in viscosity follows the first very slow polyurethane formation in the mixing zone which is accelerated by the heated air. The increase in viscosity is advantageous to the process. Moreover, the flow problems and droplet formation is prevented on vertical surfaces.
When using the two component systems of the prior art, it was necessary to work with expensive, complicated proportioning and mixing equipment, often under difficult conditions. For some time, there has been a need to replace these systems with more easily manageable polyurethane single component systems.
Prepolymers containing NCO groups in bonded form have acquired great importance as polyurethane single component systems. These prepolymers are preferably based on higher molecular weight polyhydroxyl compounds and organic polyisocyanates such as hexamethylene diisocyanates, toluene diisocyanates and diphenylmethane diisocyanates, which, following moisture contact on their upper surface, cure into urea group containing polyurethanes. When employing high moisture concentrations and thick polyurethane coatings, the curing process must be adjusted in order to avoid bubble formation so that any carbon dioxide formed can escape prior to curing.
By adding latent hardeners to the prepolymers containing NCO groups in bonded form, single component polyurethane systems are obtained which cure without the formation of carbon dioxide. The latent hardeners are blocked reaction partners for the organic polyisocyanates which, under the influence of moisture, are converted into reactive compounds. Typical latent hardeners are oxazolidines, enamines and azomethines, such as ketimines and aldimines. Polyurethane single component systems, which contain prepolymers having NCO groups in bonded form and latent hardeners, cure bubble free independent from the moisture content of the surroundings, even under water. Accordingly, the outdoor processing of such polyurethane single component systems is essentially independent of the weather.
The rate of diffusion of water into the curable polyurethane masses is indicative of the cure rate for the polyurethane single component systems. Thus, thicker polyurethane coatings cure more slowly. For this reason, thicker coating layers are difficult to apply on vertical surfaces because of coating mass run off.